Sheet molding compounds (“SMC”), bulk molding compounds (“BMC”) and thick molding compounds (“TMC”) are fiber reinforced thermosetting resin molding compositions (sometimes referred to hereinafter as “compounds” in accordance with customary practice in this field) which are widely used in industrial molding processes such as compression molding, etc. Such fiber reinforced thermosetting resin molding compounds typically comprise a curable polymer resin and a curing agent capable of causing the resin to rapidly cure when the molding compound is heated or otherwise processed to activate the curing agent.
In order to increase productivity, such fiber reinforced thermosetting resin molding compounds are often made with high activity curing agents. Unfortunately, this often leads to a corresponding decrease in shelf life relative to conventional fiber reinforced thermosetting resin molding compounds, which typically have a shelf life on the order of three months or so. To deal with this problem, it has already been proposed to microencapsulate the curing agent in a suitable protective coating or shell.
For example, WO 84/01919, the entire disclosure of which is incorporated herein by reference, describes a process for making microencapsulated curing agents for unsaturated polyester resin SMCs and BMCs in which an organic peroxide curing agent is microencapsulated in a phenol-formaldehyde resin shell. A problem with this approach, however, is that the curing agent may evolve suddenly in response to the rising temperature encountered during the molding process. This can lead to a non-uniform distribution of the curing agent in the resin being molded, which in turn can cause an irregularity of cure and hence poor surface appearance of the molded product obtained.
An alternative approach is described in JP 4175321, the entire disclosure of which is also incorporated herein by reference. In this approach, the organic peroxide curing agent is microencapsulated in gelatin or the like. Unfortunately, the shelf life of such molding compounds is unacceptably short, especially those containing styrene or analogous monomer, which is believed due to reaction of the thermosetting resin with curing agent leaking out of the microcapsules by contact with the styrene. Moreover, when the microencapsulated curing agent is dried powder, curing of the resin may be insufficient, which is believed due to the fact that the pressure created by the molding process is inadequate to cause the curing agent to evolve fully from its microcapsules.